The present invention relates to a seal for a sensor element of a gas sensor and to a method for fabricating the seal.
A previously proposed seal is described in German Published Patent Application No. 41 26 378. This seal uses a sealing element of steatite, which is pressed between two vitrified ceramic molded parts. The seal separates a section of the sensor element on the sampled-gas side from a terminal-side section of the sensor element, the terminal-side section projecting into a reference gas chamber, into which a reference gas is admitted. To manufacture the seal, an initially compressed sealing ring having the strength needed for an automatic assembly, is initially introduced into the longitudinal bore and is subsequently pressed between the two ceramic molded parts. When the sealing ring is pressed, it is transformed into steatite powder, which is then applied radially against the sealing element and against the inner bore of the housing, in the process, sealing off the sensor element in the housing. The thus formed seal has considerable residual porosity, which cannot be significantly reduced even by substantial pressing forces. This means that the steatite seal is only resistant in a limited fashion to liquid and gaseous fuel, as well as to water and water vapor. The fuel vapors present in the exhaust gas diffuse into the seal and are partially absorbed by the material of the seal. In response to heating of the gas sensor, the hydrocarbons are driven rapidly out of the seal by the high water vapor pressure. The component arriving in the reference gas chamber reacts there with the oxygen, the result being a change in the composition of the reference gas, thereby negatively influencing the sensor signal.
An improved sealing action with respect to fuel vapors is provided by a sealing arrangement described in German Patent No. 195 32 090. In this sealing arrangement, an additional sealing element made of boron nitride is pressed between two steatite sealing elements. The storage capacity of the boron nitride is many times less than that of steatite. However, to ensure a proper sealing function for this sealing arrangement, a narrowest as possible gap is required between the premolded boron nitride sealing ring and the probe housing, as well as between the boron nitride sealing ring and the sensor element. This places stringent demands on the dimensional accuracy of the individual elements.
A further specific embodiment of a seal provides for fixing the sensor element in a ceramic retaining member using a glass seal European Published Patent Application No. 706 046. A drawback of this approach, however, is that the sensor element is fixed in a substantially rigid fashion in the solidified glass melt. Depending on the temperature conditions prevailing during heating and cooling, or under the stress of external thermal shock, strain conditions are produced in the glass melt which lead to cracks in the same and act with a high level of tensile stress on the sensor element.
The object of the present invention is to fix the sensor element in position in a manner that is substantially free of forces, making it impervious to gas and fuel, and, moreover, enabling it to be integrated cost-effectively into large-scale production.
The seal according to the present invention, has the advantage of being impervious to gas, as well as to liquids, in particular to fuel, and, additionally, of being substantially temperature-resistant. The method according to the present invention is beneficial in that the manufacturing of the seal can be integrated cost-effectively in the mass production of gas sensors.
The measures delineated in the dependent claims render possible advantageous further embodiments of the seal specified in the main claim. Suitable glasses having the required low melting point are those containing lead, zinc, bismuth, or alkaline-earth metals in the form of oxides, borates, phosphates, or silicates, or a mixture thereof. An especially temperature-resistant, as well as gas- and fuel-impermeable seal is attained when the proportion of the steatite material amounts to at least 70% by weight, for example, between 70 and 95% by weight. The glass may be used, for example, with a proportion of 5 to 30% by weight. Optimal results were attained with a mixture ratio of powdered glass of 10-20% by weight and of steatite powder of 80 to 90% by weight.